According to BND documents seen by journalists, five locations around Germany are used to collect the millions of so-called 'metadata' about foreigners' phone calls.

It is not yet clear where the BND obtains this much data.

But the parliamentary committee at the Bundestag (German parliament) investigating surveillance by the US National Security Agency (NSA) has established that the BND taps satellite communications and internet cables.

One percent of the foreign phone call data is archived within Germany for later analysis over the long term – although most is deleted after a few weeks at most.

Government sources would not confirm or deny whether the BND's metadata archive has been reported to the government's Data Protection Commissioner, as is required by law.

But the spies have kept other databases secret for years without fulfilling their legal obligation to report them in the past.

And the classified files seen by Zeit journalists include instructions to agents that they should only inform the parliamentary Oversight Committee about how much data they were gathering if asked highly targeted questions.

She said of her time on the Oversight Committee that “it was always difficult to get comprehensive information.

“You can only ask about concrete methods when you already know something about them.”

The BND has been steadily stepping up its surveillance activities, and in November it emerged that it had asked the Bundestag for €300 million between 2015 and 2020 for its so-called “Strategic Technical Initiative” to suck up and analyse even more information.

Metadata do not include the content of phone calls, but information such as the time of a call, who was speaking to whom, the location the call was made from and other facts.

The fact that the content of the calls and messages themselves are not collected has been used by spies to argue that gathering metadata does not constitute surveillance.

BND agents questioned by the NSA Committee at the Bundestag have referred to metadata as “routine traffic” in a bid to make them sound harmless.

But over time, this information about a person under surveillance's movements, routine and contacts can be used to build up a picture of their life which can be used to predict their future actions.

That's why the data are so prized by intelligence agencies, including the American NSA, which uses them to predict where they should make drone strikes against suspected terrorists.

A BND agent questioned by the NSA Committee in November told the MPs that the BND sends around 500 million metadata records per month to the NSA from a single programme spying on satellite communications.

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